Molecule harvests water's hydrogen

By
Kimberly Patch,
Technology Research NewsThe
key to producing clean hydrogen energy is finding a non-polluting method
to extract pure hydrogen from its most abundant source -- water.

Researchers have been working for decades to develop catalysts that
make it possible to use energy from sunlight to extract hydrogen from water.
These materials absorb energy from photons in order to speed the rate at
which electrons combine with hydrogen in water molecules to split water
into hydrogen and oxygen.

Such catalysts are commonly made from the semiconductor materials
used to make computer chips. Researchers are working to find catalysts that
can extract energy from a greater portion of sunlight's spectrum and use
the energy to move electrons more efficiently.

Researchers from Virginia Polytechnic and State University have
developed a large molecule, or supramolecular complex, that combines sub-units
that absorb light with sub-units that accept electrons.

The researchers' supramolecular complex could be used in devices
that use light energy to extract hydrogen from water. "The hydrogen gas
would then be used as a fuel and combusted in the presence of oxygen in
air to produce water and energy, either in the form of heat for a combustion
engine or electricity from a fuel cell," Karen Brewer, an associate professor
of chemistry at Virginia Tech.

Plants carry out photosynthesis by absorbing photons and using their
energy to generate biomass. "This is accomplished through very complex pathways
in biological systems," said Brewer. Nearly all the energy we use, including
oil, coal, wood, and gas, is second-hand solar energy drawn from this biomass.
The researchers' artificial photosynthesis process bypasses the photosynthetic
plant altogether by using light to produce fuel directly.

It has been known for years that molecules containing the metal
ruthenium absorb solar light well and are capable of producing enough energy
to carry out hydrogen production, said Brewer. The stumbling block to producing
such molecules is getting light to generate two or more electrons at a time,
which is required to generate enough energy to split water, she said.

The researchers' molecule is made up of the elements rhodium, ruthenium,
chlorine, carbon, nitrogen and hydrogen.

It has light-absorbing ruthenium subunits on each end, connector
sub-units near the middle, and a reactive rhodium sub-unit in the center
that collects electrons and delivers them to water. "Each sub-unit in our
assembly performs an individual task," said Brewer. "The assembly of these
sub-units into a larger structure allows each sub-unit to perform its task
and, combined with other sub-unit's tasks, [accomplish] a more complex function,"
she said.

Figuring out how to design, prepare and use a supramolecular complex
capable of using light to collect electrons took more than a decade of work,
said Brewer. "We have worked for many years on trying to understand the
properties of these supramolecular complexes well enough that we would be
able to design the right systems to perform this complex function," she
said.

Custom-designed supramolecular complexes promise to be useful in
other areas as well. "Understanding the interactions of light with complex
molecular systems could have broad impact in a variety of processes that
involve light energy," said Brewer.

The researchers are working on adjusting the molecular design to
optimize the efficiency and long-term stability of the molecular complexes,
said Brewer. "We will be... working to optimize the system to produce the
best hydrogen production per amount of light absorbed [and] to absorb the
solar spectrum fully."

The wavelengths in sunlight range from infrared through visible
light to ultraviolet. Sunlight is about 53 percent infrared, which includes
the energy, 43 percent visible light, and 4 percent ultraviolet. The researchers'
supramolecular complex absorbs photons from with wavelengths from 200 to
600 nanometers, which range from the near infrared through most of the visible
spectrum. A nanometer is one millionth of a millimeter.

The researchers are also working on the oxygen half of the water-splitting
reaction, which could be accomplished by adding other subunits to the molecule,
said Brewer. The existing molecule generates hydrogen but leaves oxygen
bound to other elements of the reaction. Other subunits would complete the
process by freeing the oxygen.

Brewer's research colleague was Mark Elvington. They presented the
research at the 228th American Chemical Society National Meeting held in
Philadelphia on August 22 through 26, 2004. The research was funded by the
National Science Foundation (NSF) and the American Chemical Society.